Since the Digital Micromirror Device (DMD) was invented, many an optical micromirror device has been introduced. Micromirror related devices are usually light reflecting and light modulating devices. Especially controlling light and having a good quality of reflectivity is essential to the device as well as the operation of the micromirror devices. To improve the optical quality of the micromirror device, scientists and engineers have been making efforts for developing new optical coatings and new structures for micromirror devices.
Hornbeck discloses a micromirror array device with metal layer made of aluminum alloy in U.S. Pat. No. 5,083,857. Since the micromirror array was made by the aluminum alloy, the micromirror array device has a reflectivity of metal. It has a good performance of light reflecting and modulating. Even though aluminum alloy has a high reflectivity, the metal surface is degraded by oxidation. Since the metal layer was exposed to the environments, the reflectivity of the micromirror was slowly degraded by oxidation.
To enhance the optical properties of the micromirror device, anti-reflective coatings are also investigated. Some example can be found in the U.S. Pat. No. 6,282,010 to Sulzbach, and the U.S. Pat. No. 7,009,745 to Miller. In those patents, the structures under the reflective surface are coated with anti-reflective materials. Since the structure of the micromirror device was made with metal or metal alloy, the structure itself has a relatively high reflectivity. The residual light reflected from the metal surface other than reflective mirror surface made serious problems for generating images with high resolution and high quality. The anti-reflection coating for the structure enhanced the optical quality of the micromirror device. But the enhancement was not enough and the process for anti-reflective coating was complex and difficult.
By introducing wafer bonding and transparent substrates, a different structure for micromirror device was disclosed by Huibers in the U.S. Pat. No. 5,835,256. The device has a better protection for reflective mirror surface, but the fabrication of the device becomes considerably difficult with fabrication on the transparent wafer and wafer bonding of two different wafers. One more problem is that this structure experiences a thermal degradation of the reflectivity.
More recently, another micromirror array device was disclosed in U.S. Pat. No. 6,970,284 to Kim, U.S. Pat. No. 7,031,046 to Kim, U.S. Pat. No. 6,934,072 to Kim, U.S. Pat. No. 6,934,073 to Kim, U.S. Pat. No. 6,999,226 to Kim. The Micromirror Array Lens acts as a variable focus lens by controlling micromirrors in the Micromirror Array Lens. The micromirrors in the Micromirror Array Lens need a good quality of optical coating as well as protection for the micro-mechanical structures. Good quality of optical coating is closely related to the performance of the Micromirror Array Lens. Since the device acts as a lens, the high reflective surface of the micromirrors is essential to the device. Also the protection of the micro-mechanical structure is a must to have precise motion control of the micromirror device.
In the present invention, a new structure and method for enhancing optical properties as well as protection of the micro-mechanical structures. The present invention is dedicated to solve the following problems: oxidation of the metal coating, degradation of the reflective coating layer, protection of micro-mechanical structures and reflective surface from the acid and base, protection of reflective surface from severe environments, providing the anti-reflective coating for optically non-effective area, providing protective layer for reflective surface, and simplifying the process of fabrication.